Dampers, shock absorbers, brakes and clutches are known which use a fluid as the working medium to create damping forces/torques to control vibration (translational or rotational). One class of these devices are termed "controllable" and employ Electrorheological controllable fluids (ER), Electrophoretic fluids (EP), Magnetorheological fluids (MR), or Hydraulic fluids (Semi-Active Electro-mechanical), etc. Of particular interest are "controllable fluid devices" (otherwise known as field responsive devices), i.e., devices which have a contained fluid which responds to an applied field (electric or magnetic). One particular type of "controllable fluid device" is a Magnetorheological (MR) fluid device. MR fluid devices may be of the rotary or linear-acting (translational) variety, such as controllable fluid MR dampers, MR brakes, or MR clutches. They employ a controllable MR fluid comprised of fine soft-magnetic particles disbursed within a liquid carrier. Typical particles include carbonyl iron having various shapes, but which are preferably approximately spherical, and which exhibit mean dimensions of about 0.1 to 500 .mu.m, and more preferably between 1 and 100 .mu.m. The carrier fluids include various known hydraulic oils, silicone oils, and the like.
MR fluids exhibit a "thickening" behavior (a rheology change), sometimes referred to as an "apparent viscosity change", upon being exposed to a magnetic field of sufficient strength. The higher the magnetic field strength exposed to the MR fluid, the higher the damping force that can be achieved within the particular MR device. Examples of prior art MR fluids can be found in commonly assigned WO 94/10694. Notably, MR fluid devices provide ease of controllability through simple variations in electrical current supplied thereto. In particular, MR fluids and devices have demonstrated excellent durability as compared to ER devices (ER fluids exhibit a rheology change upon being exposed to an electric field) and simplicity previously unachievable with controllable semi-active hydraulic devices (which include electro-mechanically actuated valves).
Descriptions of prior art MR controllable fluid devices can be found in commonly assigned U.S. Ser. No. 08/674,179 entitled "Controllable Vibration Apparatus", and U.S. Pat. Nos. 5,547,049, 5,492,312, 5,398,917, 5,284,330, and 5,277,281. Notably, these devices provide real time-variable control forces.
Generally known cab suspensions comprise a cab, a chassis or frame, air springs flexibly supporting the cab relative to the frame, and a plurality of "passive" hydraulic dampers (commonly known shock absorbers) for providing stabilizing damping forces between the cab and frame. On one hand, the air springs, which provide low spring rates are needed for good isolation. See, for example, U.S. Pat. No. 4,029,305 to Schubert, which teaches a pneumatic isolator and suspension system. On the other hand, the hydraulic dampers must be highly damped to limit motions upon encountering large transient loads between the cab and frame, such as due to cab roll. Therefore, by design, the ride cannot be as good as might be achieved if lower dampiung rates could be used. In short, the need for low stiffness to achieve excellent vibration isolation has the drawback of allowing large transient motions upon encountering large transient forces, which can cause instability.
To solve this problem, the afore-mentioned prior art systems added highly damped passive dampers, thereby generally degrading ride quality. This is due to the fact that very high damping rates in the passive dampers cannot be used without having the concomitant result of producing a harsh ride. Furthermore, although these systems provide good vibration isolation, they cannot adequately damp large transient motions. For example, suspension systems including passive dampers still allow large cab roll angles during cornering or abrupt maneuvers, especially on trucks having a high center of gravity. Moreover, large pitch (dive) angles may still occur during abrupt braking, especially on cab-over truck designs. Furthermore, the cab may lurch upon rapid acceleration or when encountering rough terrain (pot holes, uneven or dirt parking lots, or generally rough roads). Therefore, there is a need for a simple, cost effective suspension system that can control these transient motions without degrading ride.
Some systems have been developed which use pneumatics to reposition a flow restrictor valve in the damper to control the level of damping from a high damped level to a low damped level by adjusting the valve position in the damper from a high damping state to a low damping state. One such system, called the LINK RIDE COMMAND driver controlled ride system is available from Link Mfg. Ltd. of Sioux Center, IA. Other known systems include a moveable damping valve which closes off fluid flow in one direction is taught in U.S. Pat. No. 4,506,751 to Stephens. Notably, in these systems it is difficult to fully lock up the cab upon encountering such transient conditions. Further, they have moving parts in the valve which tend to be expensive and unreliable.
More sophisticated fully active systems have also been employed to solve pitching due to braking and cab roll. Certain of these systems can be found in U.S. Pat. No. 4,483,409 to Fun, U.S. Pat. No. 5,044,455 to Tecco et al., and U.S. Pat. No. 5,555,501 to Furihata et al. While fully active systems may solve some of these problems, they are very expensive, require large amounts of power, and are very complicated. Therefore, there is a long felt and unfulfilled need for a cab suspension system which can solve the afore-mentioned problems associated with the prior art passively-damped suspension systems, yet which is cost effective, has low power requirements, and is simple to retrofit to existing platforms.